Wiresaw cutting is the dominant method for slicing ingots into thin wafers for use in the integrated circuits and photo-voltaic device (PV) industries. This method is also commonly used for wafering substrates of other materials, such as sapphire, silicon carbide, or ceramic substrates. A wiresaw typically has a web of fine metal wires, or a wireweb, where the individual wires have a diameter around 0.15 mm and are arranged parallel to each other, at a distance of 0.1 to 1.0 mm, through a series of spools, pulleys and wire guides. Slicing, or cutting, is accomplished by contacting the workpiece (e.g. a silicon ingot) with a moving wire to which an abrasive cutting fluid cutting fluid has been applied.
Conventional wiresaw abrasive slurries typically comprise a carrier and abrasive particles combined by mixing in a ratio of about 1:1 by weight. The carrier is a liquid that provides lubrication and cooling, such as a mineral oil, kerosene, polyethylene glycol, polypropylene glycol or other polyalkylene glycols. The liquid carrier also holds the abrasive to the wire so that the abrasive can contact the workpiece. The abrasive is typically a hard material such as silicon carbide particles.
As noted above, the carrier can be a non-aqueous substance such as mineral oil, kerosene, or poly(alkylene glycol) materials such as a poly(ethylene glycol), or a poly(propylene glycol). Non-aqueous carriers can have several disadvantages, however. For example, non-aqueous carriers can have limited shelf-life because of colloidal instability, and also can exhibit poor heat transfer characteristics.
Aqueous carriers also can be used for wiresaw cutting processes; however, such carriers have certain known disadvantages, as well. For example, during the wiresaw cutting process, a portion of the material being cut is removed. This material, called kerf, gradually accumulates in the cutting fluid cutting fluid. In the process of wiresawing silicon and other water oxidizable substrates, the kerf can become oxidized by oxygen or water. In an aqueous cutting fluid, oxidation of a water oxidizable workpiece by water produces hydrogen. The presence of hydrogen in the cutting fluid composition can disrupt the cutting fluid distribution on the wire web and reduce the cutting performance of the wiresaw. The creation and potential accumulation of hydrogen can also be hazardous in a manufacturing environment.
Aqueous cutting fluids can contain multiple functional chemical components, such as surfactants, antifoaming agent, oxidizing agents, pH buffers, thickening agents, and the like, which can degrade due to conditions (e.g., shear, frictional heat, etc.) that can arise during the cutting of a workpiece. For example, some components may become adsorbed onto the kerf generated during the cutting operation, certain polymeric materials may break down into lower molecular mass forms due to shear experienced at the interface between the cutting wires and the workpiece, redox reactions between the workpiece (e.g., a silicon ingot) and a component of the cutting fluid; etc. Chemical changes in the cutting fluid that occur over time can lead to non-uniform physical properties of the cutting fluid during the cutting of a workpiece. This can reduce efficiency of the cutting process or lead to failure of the cutting wires, and the like.
It is desirable for the chemical composition and the physical properties of the cutting fluid (e.g., viscosity, pH, foam level, etc.) to remain relatively constant for the duration of the cutting process. This can be problematic when aqueous cutting fluids are utilized, due to the decomposition and instability issues described above. Accordingly, there is an ongoing need for methods of maintaining the integrity of the chemical composition and/or properties of an aqueous cutting fluid throughout a cutting operation. The present invention fulfills this need.